1. Field of the Invention
The invention relates to a method and apparatus for precisely measuring a range of relatively low differential pressures existing between known, relatively high pressure sources. One or more scanner modules are utilized, each incorporating a plurality of differential pressure sensors, and each adapted to house ports and valving for internally coupling the pressure sources to each of the sensors.
2. Description of the Prior Art
It is known to measure differential pressures by employing a so-called patch panel having a single reference port and a pair of calibration ports.
The reference port in the patch panel is adapted to be coupled to a known reference pressure. The port is selectively coupled to a manifold in the patch panel through suitable valving in the patch panel. Likewise, the calibration ports in the patch panel are adapted to be coupled to known calibration pressures for coupling to the manifold.
The manifold enables application of reference and calibration pressures to sensor outlets that are located in the patch panel. The sensor outlets are connectible to differential pressure sensors in one or more separate scanner modules. Any sensor outlets in the patch panel in excess of the number of sensors in the scanner modules must be temporarily plugged.
This type of system is sometimes referred to as a high line, low differential pressure measurement and calibration system. High line pressures are involved, but it is the dynamic measurement of the relatively low pressure differences existing between the high line pressures which is of interest.
For example, in the design of gas turbines, it is important to measure and compare the pressure differentials which exist across the compressor stages of the turbines. Although it is possible to use a pair of very high accuracy pressure sensors, and measure the absolute pressures present at each of the adjacent turbine stages to determine the pressure difference, this technique does not produce accurate results.
More particularly, if the high pressures at the two stages are in the range of 100 pounds per square inch (psi), and if a pair of high accuracy sensors are selected having a full scale deflection of perhaps 100 psi, these sensors might, for example, yield readings of 90 and 95 psi, respectively. The differential pressure of 5 psi would represent only a small part of the full scale deflection of the sensors. Detecting and precisely measuring such relatively small differential pressures is difficult where the sensor is being deflected over a relatively wide range by relatively high line pressures.
Use of the present system of high line low differential pressure measurement does away with the need for relatively expensive sensors to accurately measure the high line pressures at adjacent turbine stages. Instead, a number of relatively inexpensive differential pressure sensors are selected which have a full scale deflection approximating the maximum differential pressure expected, thus achieving the accuracy which full scale deflection affords. These measure the differences between, for example, a known, measured high line pressure taken off one turbine stage, and a number of unknown high line pressures taken off points adjacent the turbine stage.
A differential pressure sensor which is preferred for this purpose is one which employs a piezoresistive sensor component. As is well known to those skilled in the art, such a component is typically used in circuit with a suitable control means, and is responsive to applied pressure to modulate a control signal applied to it by the control means. The magnitude and polarity of the signal changes according to the magnitude of the applied pressure, and according to the side of the component to which the pressure is applied.
The high pressure reference pressure is tapped off from one turbine stage and is precisely measured by independent means so that it is known. This known high pressure is then applied to the so-called negative side of the piezoresistive component. When the unknown or test pressure, which is also a high pressure, is applied to the opposite or positive side of the component, the sensor will provide a differential pressure.
However, the piezoresistive sensor described is characterized by differences in the configuration and volume of the spaces adjacent the positive and negative sides of the sensor component. Consequently, when equal high line pressures are applied to the opposite sides of the sensor, an "off-zero" indication results. Therefore, a line zero calibration of the sensor must be performed in order to have the sensor provide a zero indication upon the application of such equal pressures. Calibration procedures well known in the art involve the application of a range of positive pressures to one side of each of the sensor components, followed by establishment of zero (ambient) pressure at both sides, and next followed by the application of negative pressures to the other sides of the components. The term "zero" as used herein is intended to mean ambient pressure.
This allows development of a set of calibration coefficients which can be incorporated in suitable computer software programs to compensate for off-zero indications that occur when equal pressures are applied to the opposite sides of sensor components. Such calibration coefficients effectively establish a zero pressure differential whenever the same high line reference pressure is applied to both sides of a sensor. Then, under test conditions, application of a high line reference pressure and an equal test pressure, respectively, to the opposite sides of a sensor will yield a zero differential pressure reading.
As will be seen, such a calibration is a dynamic process which is done during the running of the turbine tests. This factors in any changes which may occur in the turbine stage reference pressure so that the accuracy of the differential pressure readings is relatively constant.
The foregoing system works satisfactorily for applications such as turbine testing which involve a large number of sensors, but it lacks operational flexibility where lesser numbers of sensors are involved, and where different ranges of reference and calibration pressures are to be used.
In the turbine testing system, only a single high reference pressure can be applied to a patch panel through its reference port for application to the sensors in a separate scanner module or modules. Another patch panel must be used if a different high reference pressure is to be applied to the sensors in the associated scanner modules.
Likewise, only one level of calibration pressure can be applied to a patch panel through its calibration ports for application to the opposite sides of the sensors in the separate scanner modules. If use of another calibration pressure is necessary for a further set of sensors, a second patch panel must be used to enable application of the different calibration pressure to the opposite sides of the sensors in the additional scanner module or modules. In other words, the number of reference and calibration pressure sources that can be used is restricted to the number of patch panels that are used.
It would be desirable in test systems using several scanner modules to enable application of different reference and calibration pressures to the sensors of separate scanner modules, respectively, without any need for patch panels and the multiplicity of pneumatic lines that connect them to such patch panels.